that is the way it used to be, then they made some new url things and if you do it right you no longer need to do that. Your theme probably does not use the new url variables so it is starting off at the root when it really should be at the baseurl.
The way to get around this used to be to have 2 config files - the 2nd one being for local/dev and it would only have baseurl: set to nothing, this would override the actual baseurl in the main config. Then when running locally you would change your command to include both(?) configs with the dev one second. Sounds complicated but it was pretty easy.
Some info on the relative.url and absolute_url variables here: